As per a recent request, I will write a bit on Celtic mythology. I will warn at the outset that the Celts did not leave a relatively neat and orderly mythology like the Greek, Romans, Norse, or even the Egyptians. They also did not have one set of legends for all Celts. Of all the Celtic peoples the Irish were independent longest, and their mythology the most thoroughly laid out. We will begin with them.
According to the oldest oral memories, the Fomorians were the first settlers on Ireland. Little is known of them apart from that. When Partholon and his people came to Ireland some time later they managed to defeat them for control of the island. However, after a few years there was a plague and all of Partholon’s people died.
Nemed and his people came next. They also conquered the Fomorians. However, when Nemed died his people were driven from power. When they attempted to revolt a huge wave overwhelmed Nemed’s followers. Only thirty survived, and they scattered throughout the world.
The Fir Bolg were the next invaders. However, the Fomorians were nowhere to be found they simply settled the island. Eventually the Tuatha dé Danaan, survivors of Nemed’s people, returned to the island. Through conquest and then intermarriage with the Fomorians (now emerged from wherever they had hidden), the Tuatha dé Danaan defeated the Fir Bolg.
The last invasion was of the Milesians (the present Gaelic population). They defeated the intermixed population and made a truce in which the land would be divided in half. The greatest Gaelic poet was asked to decide how that would best be done. He gave the land above the earth to his own people and that underground to the Tuatha dé Danaan. Defeated and outwitted, the Tuatha dé Danaan went underground and now inhabit only the sidh, the fairy hills.
The Tuatha dé Danaan are the fairies of Irish lore, beings of tremendous power who like to bait humans into the Celtic Otherworld. They are also the gods of Irish Mythology. And this is where Celtic Mythology falls in on itself. Lug led the Tuatha against the Fomorians, but historians have him dead long before the Milesians invaded. Another king, the sea-god Manannan, led the Tuatha dé Danaan underground after the Milesians arrived. And yet, when Conn of the Hundred Battles is first awarded the high-kingship centuries later it is Lug who arranges it.
It is an unfortunate fact in the preservation of Celtic Mythology that monks were largely responsible for all extant materials. They had an unfortunate habit of bending time and space to make native myths fit with the established chronology of their own religion. That makes Celtic mythology particularly difficult for two reasons. For one, we can be reasonably certain that Lug did not die according to Celtic legend because he was a god. But we cannot be positive. The Celts may well have introduced some aspect of the migration of souls to his story so that his body died and his soul transferred to another, essentially allowing him to be immortal in a different way.
Second, there is the question of the Tuatha dé Danaan’s retreat in the face of the Milesians. If the Tuatha dé Danaan were possessed of godlike powers, why would they need to? If they simply gave up their land to the Milesians, what was their reasoning?
The Fomorians are equally baffling. They seem to have been the original inhabitants and seem to have had some supernatural powers that made them somehow superior to every race they came up against. We know almost nothing of them, however. Likely they fall into the same category as the Jotuns of the North and the Pelagians of prehistoric Greece. They were a conquered people whose technology or philosophy was never fully understood by the conquerors and because of that the conquerors maintained a respect for the conquered.